Golf ball

ABSTRACT

A golf ball  2  has a plurality of dimples  10  on a surface thereof. Each dimple  10  has a center portion  16  and an edge portion  18 . The center portion  16  has a bowl shape. The edge portion  18  has a ring shape. The edge portion  18  is smoothly continuous with the center portion  16 . A cross-sectional shape of the edge portion  18  is a curved line that is convex outward in a radial direction of the golf ball  2 . A ratio P 1  of a number N 1  of the dimples each having an edge portion  18  that is adjacent to another edge portion  18  such that the edge portion  18  shares a boundary line with the other edge portion  18 , to a total number N of the dimples, is not less than 50%.

This application claims priority on Patent Application No. 2018-162661filed in JAPAN on Aug. 31, 2018. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to golf balls. Specifically, the presentinvention relates to dimple patterns of golf balls.

Description of the Related Art

The face of a golf club has a loft angle. When a golf ball is hit withthe golf club, backspin due to the loft angle occurs in the golf ball.The golf ball flies with the backspin.

Golf balls have a large number of dimples on the surfaces thereof. Thedimples disturb the air flow around the golf ball during flight to causeturbulent flow separation. This phenomenon is referred to as“turbulization”. Due to turbulization, separation points of the air fromthe golf ball shift backwards leading to a reduction of drag. Theturbulization promotes the displacement between the separation point onthe upper side and the separation point on the lower side of the golfball, which results from the backspin, thereby enhancing the lift forcethat acts upon the golf ball. The reduction of drag and the enhancementof lift force are referred to as a “dimple effect”. Excellent dimplesefficiently disturb the air flow. Excellent dimples produce a longflight distance.

JP2010-88640 discloses a golf ball having a random dimple pattern. Thecontour of each dimple is not circular and is also not polygonal. In thegolf ball, the area of a land is small.

JPH8-276035 discloses a golf ball having a large number of dimples and alarge number of channels. Each channel connects a dimple to anotherdimple adjacent to this dimple. The channel contributes to reduction ofa land area.

JP2005-34366 discloses a golf ball having multi-surface dimples. Eachmulti-surface dimple has a concave portion and a plurality of inclinedportions surrounding the concave portion. In the golf ball, the area ofa land is very small.

A golf ball having a land with a small area has excellent aerodynamiccharacteristic. However, with a conventional dimple pattern, the drag isnot sufficiently low. Golf players desire further improvement in flightdistance. In light of flight performance, there is room for improvementof dimples.

An object of the present invention is to provide a golf ball havingexcellent flight performance.

SUMMARY OF THE INVENTION

A golf ball according to the present invention has a plurality ofdimples on a surface thereof. Each dimple has a center portion having abowl shape, and an edge portion smoothly continuous with the centerportion and having a ring shape. A cross-sectional shape of the edgeportion is a curved line that is convex outward in a radial direction ofthe golf ball. A ratio P1 of a number N1 of the dimples each having anedge portion that is adjacent to another edge portion such that the edgeportion shares a boundary line with the other edge portion, to a totalnumber N of the dimples, is not less than 50%.

When the golf ball according to the present invention flies, the liftforce coefficient and the drag coefficient are appropriate. The golfball has excellent flight performance.

Preferably, a land ratio PL of the surface of the golf ball is notgreater than 12%.

Preferably, a cross-sectional shape of the edge portion is a circulararc having a curvature radius CR of not less than 1.25 mm.

Preferably, a total volume of the dimples is not less than 520 mm³ andnot greater than 630 mm³.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a golf ball according toan embodiment of the present invention;

FIG. 2 is an enlarged front view of the golf ball in FIG. 1;

FIG. 3 is an enlarged cross-sectional view of a part of the golf ball inFIG. 1;

FIG. 4 is an enlarged plan view of a dimple of the golf ball in FIG. 1;

FIG. 5 is an enlarged view of a part of the surface of the golf ball inFIG. 1;

FIG. 6 is a front view of a golf ball according to Example 2 of thepresent invention;

FIG. 7 is a front view of a golf ball according to Example 3 of thepresent invention;

FIG. 8 is a front view of a golf ball according to Example 4 of thepresent invention;

FIG. 9 is a front view of a golf ball according to Example 5 of thepresent invention;

FIG. 10 is a front view of a golf ball according to Example 6 of thepresent invention;

FIG. 11 is a front view of a golf ball according to Example 7 of thepresent invention;

FIG. 12 is a front view of a golf ball according to Example 8 of thepresent invention;

FIG. 13 is a front view of a golf ball according to Example 9 of thepresent invention;

FIG. 14 is a front view of a golf ball according to Example 10 of thepresent invention;

FIG. 15 is a front view of a golf ball according to Example 11 of thepresent invention;

FIG. 16 is a front view of a golf ball according to Comparative Example1;

FIG. 17 is a front view of a golf ball according to Comparative Example2;

FIG. 18 is a front view of a golf ball according to Example 12 of thepresent invention; and

FIG. 19 is a front view of a golf ball according to Example 13 of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based onpreferred embodiments with appropriate reference to the drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6positioned outside the core 4, and a cover 8 positioned outside the midlayer 6. The golf ball 2 has a large number of dimples 10 on the surfacethereof. Of the surface of the golf ball 2, a part other than thedimples 10 is a land 12. The golf ball 2 includes a paint layer and amark layer on the external side of the cover 8, but these layers are notshown in the drawing.

The golf ball 2 preferably has a diameter of not less than 40 mm and notgreater than 45 mm. From the viewpoint of conformity to the rulesestablished by the United States Golf Association (USGA), the diameteris particularly preferably not less than 42.67 mm. In light ofsuppression of air resistance, the diameter is more preferably notgreater than 44 mm and particularly preferably not greater than 42.80mm.

The golf ball 2 preferably has a weight of not less than 40 g and notgreater than 50 g. In light of attainment of great inertia, the weightis more preferably not less than 44 g and particularly preferably notless than 45.00 g. From the viewpoint of conformity to the rulesestablished by the USGA, the weight is particularly preferably notgreater than 45.93 g.

The core 4 is formed by crosslinking a rubber composition. Examples ofthe base rubber of the rubber composition include polybutadienes,polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-dienecopolymers, and natural rubbers. Two or more rubbers may be used incombination. In light of resilience performance, polybutadienes arepreferable, and high-cis polybutadienes are particularly preferable.

The core 4 may be formed from a resin composition. The core 4 may beformed from a mixture of a rubber composition and a resin composition. Aresin composition that will be described later for the mid layer 6 orthe cover 8 can be used for the core 4.

The rubber composition of the core 4 includes a co-crosslinking agent.Examples of preferable co-crosslinking agents in light of resilienceperformance include zinc acrylate, magnesium acrylate, zincmethacrylate, and magnesium methacrylate. The rubber compositionpreferably includes an organic peroxide together with a co-crosslinkingagent. Examples of preferable organic peroxides include dicumylperoxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide.

The rubber composition of the core 4 may include additives such as afiller, sulfur, a vulcanization accelerator, a sulfur compound, ananti-aging agent, a coloring agent, a plasticizer, and a dispersant. Therubber composition may include a carboxylic acid or a carboxylate. Therubber composition may include synthetic resin powder or crosslinkedrubber powder.

The core 4 has a diameter of preferably not less than 30.0 mm andparticularly preferably not less than 38.0 mm. The diameter of the core4 is preferably not greater than 42.0 mm and particularly preferably notgreater than 41.5 mm. The core 4 may have two or more layers. The core 4may have a rib on the surface thereof. The core 4 may be hollow.

The mid layer 6 is formed from a resin composition. A preferable basepolymer of the resin composition is an ionomer resin. Examples ofpreferable ionomer resins include binary copolymers formed with anα-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms. Examples of other preferable ionomer resins include ternarycopolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate esterhaving 2 to 22 carbon atoms. For the binary copolymer and the ternarycopolymer, preferable α-olefins are ethylene and propylene, whilepreferable α,β-unsaturated carboxylic acids are acrylic acid andmethacrylic acid. In the binary copolymer and the ternary copolymer,some of the carboxyl groups are neutralized with metal ions. Examples ofmetal ions for use in neutralization include sodium ion, potassium ion,lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, andneodymium ion.

Instead of an ionomer resin, the resin composition of the mid layer 6may include another polymer. Examples of the other polymer includepolystyrenes, polyamides, polyesters, polyolefins, and polyurethanes.The resin composition may include two or more polymers.

The resin composition of the mid layer 6 may include a coloring agentsuch as titanium dioxide, a filler such as barium sulfate, a dispersant,an antioxidant, an ultraviolet absorber, a light stabilizer, afluorescent material, a fluorescent brightener, and the like. For thepurpose of adjusting specific gravity, the resin composition may includepowder of a metal with a high specific gravity such as tungsten,molybdenum, and the like.

The mid layer 6 has a thickness of preferably not less than 0.2 mm andparticularly preferably not less than 0.3 mm. The thickness of the midlayer 6 is preferably not greater than 2.5 mm and particularlypreferably not greater than 2.2 mm. The mid layer 6 has a specificgravity of preferably not less than 0.90 and particularly preferably notless than 0.95. The specific gravity of the mid layer 6 is preferablynot greater than 1.10 and particularly preferably not greater than 1.05.The mid layer 6 may have two or more layers.

The cover 8 is formed from a resin composition. A preferable basepolymer of the resin composition is a polyurethane. The resincomposition may include a thermoplastic polyurethane or may include athermosetting polyurethane. In light of productivity, the thermoplasticpolyurethane is preferable. The thermoplastic polyurethane includes apolyurethane component as a hard segment, and a polyester component or apolyether component as a soft segment.

The polyurethane has a urethane bond within the molecule. The urethanebond can be formed by reacting a polyol with a polyisocyanate.

The polyol, which is a material for the urethane bond, has a pluralityof hydroxyl groups. Low-molecular-weight polyols andhigh-molecular-weight polyols can be used.

Examples of an isocyanate for the polyurethane component includealicyclic diisocyanates, aromatic diisocyanates, and aliphaticdiisocyanates. Alicyclic diisocyanates are particularly preferable.Since an alicyclic diisocyanate does not have any double bond in themain chain, the alicyclic diisocyanate suppresses yellowing of the cover8. Examples of alicyclic diisocyanates include 4,4′-dicyclohexylmethanediisocyanate (H₁₂MDI), 1,3-bis(isocyanatomethyl)cyclohexane (H₆XDI),isophorone diisocyanate (IPDI), and trans-1,4-cyclohexane diisocyanate(CHDI). In light of versatility and processability, H₁₂MDI ispreferable.

Instead of a polyurethane, the resin composition of the cover 8 mayinclude another polymer. Examples of the other polymer include ionomerresins, polystyrenes, polyamides, polyesters, and polyolefins. The resincomposition may include two or more polymers.

The resin composition of the cover 8 may include a coloring agent suchas titanium dioxide, a filler such as barium sulfate, a dispersant, anantioxidant, an ultraviolet absorber, a light stabilizer, a fluorescentmaterial, a fluorescent brightener, and the like.

The cover 8 has a thickness of preferably not less than 0.2 mm andparticularly preferably not less than 0.3 mm. The thickness of the cover8 is preferably not greater than 2.5 mm and particularly preferably notgreater than 2.2 mm. The cover 8 has a specific gravity of preferablynot less than 0.90 and particularly preferably not less than 0.95. Thespecific gravity of the cover 8 is preferably not greater than 1.10 andparticularly preferably not greater than 1.05. The cover 8 may have twoor more layers.

The golf ball 2 may include a reinforcing layer between the mid layer 6and the cover 8. The reinforcing layer firmly adheres to the mid layer 6and also to the cover 8. The reinforcing layer suppresses separation ofthe cover 8 from the mid layer 6. The reinforcing layer is formed from apolymer composition. Examples of the base polymer of the reinforcinglayer include two-component curing type epoxy resins and two-componentcuring type urethane resins.

As shown in FIG. 2, the golf ball 2 has dimples A to E. The dimples Aare larger than the dimples B. The dimples B are larger than the dimplesC. The dimples C are larger than the dimples D. The dimples D are largerthan the dimples E. The number of the dimples A is 60; the number of thedimples B is 158; the number of the dimples C is 72; the number of thedimples D is 36; and the number of the dimples E is 12. The total numberof the dimples 10 is 338. A dimple pattern is formed by these dimples 10and the land 12.

FIG. 3 shows a cross section of the golf ball 2 along a plane passingthrough the central point of a dimple 10 and the central point of thegolf ball 2. In FIG. 3, the top-to-bottom direction is the depthdirection of the dimple 10. In FIG. 3, an alternate long and two shortdashes line 14 represents a phantom sphere. The surface of the phantomsphere 14 is the surface of the golf ball 2 when it is postulated thatno dimple 10 exists. The diameter of the phantom sphere 14 is equal tothe diameter of the golf ball 2. The dimple 10 is recessed from thesurface of the phantom sphere 14. The land 12 coincides with the surfaceof the phantom sphere 14. In FIG. 3, reference character EP representsan edge point of the dimple 10, and reference character JP represents ajoint point (described in detail later).

FIG. 4 is an enlarged plan view of a dimple 10 of the golf ball 2 inFIG. 1. In FIG. 4, reference character EL represents an edge line of thedimple 10. The edge line EL is a set of a large number of edge points EP(see FIG. 3). Of the surface of the golf ball 2, a part surrounded bythe edge line EL is the dimple 10. Of the surface of the golf ball 2,the outside of the edge line EL is the land 12 (see FIG. 2).

The dimple 10 has a center portion 16 and an edge portion 18. In FIG. 4,reference character JL represents a joint line. The joint line JL is aset of a large number of joint points JP (see FIG. 3). Of the dimple 10,a part surrounded by the joint line JL is the center portion 16. Of thedimple 10, a part between the edge line EL and the joint line JL is theedge portion 18.

As shown in FIGS. 3 and 4, the center portion 16 has a bowl shape. Across-sectional shape of the center portion 16 is a curved line that isconvex inward in the radial direction of the golf ball 2. In the presentembodiment, the cross-sectional shape of the center portion 16 is acircular arc. The cross-sectional shape of the center portion 16 may bea combination of a plurality of circular arcs. The cross-sectional shapeof the center portion 16 may be a curved line other than a circular arc.The cross-sectional shape of the center portion 16 may be partiallyconvex outward. In the present embodiment, the contour of the centerportion 16 (that is, the joint line JL) is a circle.

As shown in FIGS. 3 and 4, the edge portion 18 has a ring shape. Across-sectional shape of the edge portion 18 is a curved line that isconvex outward in the radial direction of the golf ball 2. In thepresent embodiment, the cross-sectional shape of the edge portion 18 isa circular arc. The cross-sectional shape of the edge portion 18 may bea combination of a plurality of circular arcs. The cross-sectional shapeof the edge portion 18 may be a curved line other than a circular arc.

In the present embodiment, the curved line of the edge portion 18 istangent to a curved line of the phantom sphere 14. In other words, theedge portion 18 is smoothly continuous with the phantom sphere 14.Therefore, in the actual golf ball 2, the edge line EL is not clearlyviewed as an edge.

The curved line of the cross-section of the center portion 16 and thecurved line of the cross-section of the edge portion 18 are tangent toeach other at the joint point JP. In other words, the edge portion 18 issmoothly continuous with the center portion 16. Therefore, in the actualgolf ball 2, the joint line JL is not clearly viewed as an edge.

FIG. 5 is an enlarged view of a part of the surface of the golf ball 2in FIG. 1. FIG. 5 shows three dimples 10 a to 10 c. In the presentinvention, when a straight line connecting the centers of two dimples 10to each other does not intersect any other dimple 10, these two dimples10 are referred to as dimples 10 near each other. A straight lineconnecting the center of the dimple 10 a and the center of the dimple 10b to each other does not intersect any other dimple 10. Therefore, thedimple 10 b is a dimple near the dimple 10 a. A straight line connectingthe center of the dimple 10 a and the center of the dimple 10 c to eachother does not intersect any other dimple 10. Therefore, the dimple 10 cis a dimple near the dimple 10 a.

An edge portion 18 a of the dimple 10 a is not separated from the edgeportion 18 b of the dimple 10 b. The edge portion 18 a is adjacent tothe edge portion 18 b such that the edge portion 18 a shares a boundaryline 20 with the edge portion 18 b. In the present invention, a dimple10 having an edge portion 18 that is adjacent to the edge portion 18 ofanother dimple 10 such that the edge portion 18 shares a boundary line20 with the edge portion 18 of the other dimple 10, is defined to be“adjacent to” the other dimple 10. The dimple 10 b is adjacent to thedimple 10 a. In the present embodiment, the boundary line 20 is a curvedline that is convex outward in the radial direction of the golf ball 2.The edge portion 18 a of the dimple 10 a is separated from an edgeportion 18 c of the dimple 10 c. The dimple 10 c is not adjacent to thedimple 10 a.

The dimple 10 b is a dimple near the dimple 10 a and is a dimpleadjacent to the dimple 10 a. The dimple 10 c is a dimple near the dimple10 a but is not a dimple adjacent to the dimple 10 a.

Each dimple 10 having the edge portion 18 is larger than a dimple havingonly a center portion 16. In the golf ball 2 having the dimples 10, thearea of the land 12 is small. With the golf ball 2, a low dragcoefficient can be achieved. In the golf ball 2, each center portion 16does not interfere with any other dimple 10. Therefore, the shape of thecenter portion 16 is not influenced by any other dimple 10. The centerportion 16 can contribute to a low drag coefficient. The golf ball 2 hasexcellent flight performance.

The effect by the dimple 10 is achieved particularly upon a shot with adriver. When the golf ball 2 is hit with a driver, a low dragcoefficient is obtained in a trajectory after the peak.

In the present invention, a ratio P1 is calculated by the followingmathematical formula.

P1=(N1/N)*100

In this mathematical formula, N1 is the number of the dimples 10 each ofwhich is adjacent to one or more dimples 10, and N is the total numberof the dimples 10.

The ratio P1 is preferably not less than 50%. The golf ball 2 in whichthe ratio P1 is not less than 50% has excellent flight performance. Fromthis viewpoint, the ratio P1 is more preferably not less than 70% andparticularly preferably not less than 90%. The ratio P1 is ideally 100%.

In the present invention, a ratio P2 is calculated by the followingmathematical formula.

P2=(N2/N)*100

In this mathematical formula, N2 is the number of the dimples 10 eachhaving a center portion 16 that interferes with any other dimple 10, andN is the total number of the dimples 10.

The ratio P2 is preferably not greater than 20%. The golf ball 2 inwhich the ratio P2 is not greater than 20% has low drag. From thisviewpoint, the ratio P2 is more preferably not greater than 10% andparticularly preferably not greater than 5%. The ratio P2 is ideally 0%.

The number of the dimples 10 adjacent to one dimple 10 is preferably notless than 2, more preferably not less than 3, and particularlypreferably not less than 4, on average. This number is preferably notgreater than 8.

A land ratio PL of the surface of the golf ball 2 is preferably notgreater than 12%. The land ratio PL is the ratio of the area of the land12 to the surface area of the phantom sphere 14. The golf ball 2 inwhich the land ratio PL is not greater than 12% has excellent flightperformance. From this viewpoint, the land ratio PL is more preferablynot greater than 10% and particularly preferably not greater than 8%.

In FIG. 3, an arrow CR represents the curvature radius of the circulararc of the edge portion 18. The curvature radius CR is preferably notless than 1.25 mm. The edge portion 18 having a curvature radius CR ofnot less than 1.25 mm contributes to a low drag coefficient. From thisviewpoint, the curvature radius CR is preferably not less than 1.50 mmand particularly preferably not less than 1.75 mm. From the viewpointthat the center portion 16 can have a sufficient volume, the curvatureradius CR is preferably not greater than 4.0 mm.

In each dimple 10, the curvature radius CR is preferably within theabove range. In only some of the dimples 10, the curvature radius CR maybe within the above range. The ratio of the number of the dimples 10each having a curvature radius CR within the above range, to the totalnumber N of the dimples 10, is preferably not less than 50%, morepreferably not less than 70%, and particularly preferably not less than90%.

In FIG. 3, a double headed arrow Dp1 represents the depth of the dimple10. The depth Dp1 is the distance between the deepest part of the dimple10 and the surface of the phantom sphere 14. In FIG. 3, a double headedarrow Dp2 represents the depth of the center portion 16. The depth Dp2is the distance between the deepest part of the dimple 10 and a planeincluding the joint line JL. From the viewpoint that the center portion16 and the edge portion 18 both contribute to aerodynamiccharacteristic, the ratio of the depth Dp2 to the depth Dp1 ispreferably not less than 20% and not greater than 80%, more preferablynot less than 30% and not greater than 70%, and particularly preferablynot less than 35% and not greater than 65%.

From the viewpoint that rising of the golf ball 2 during flight issuppressed, the depth Dp1 of the dimple 10 is preferably not less than0.10 mm, more preferably not less than 0.13 mm, and particularlypreferably not less than 0.15 mm. From the viewpoint that dropping ofthe golf ball 2 during flight is suppressed, the depth Dp1 is preferablynot greater than 0.65 mm, more preferably not greater than 0.60 mm, andparticularly preferably not greater than 0.55 mm.

From the viewpoint that a sufficiently low land ratio PL is achieved,the total number of the dimples 10 is preferably not less than 250, morepreferably not less than 280, and particularly preferably not less than300. From the viewpoint that each dimple 10 can contribute toturbulization, the total number is preferably not greater than 450, morepreferably not greater than 410, and particularly preferably not greaterthan 390.

In the present invention, the “volume V of the dimple” means the volumeof a portion surrounded by the surface of the phantom sphere 14 and thesurface of the dimple 10. The total volume TV of the dimples 10 ispreferably not less than 520 mm³ and not greater than 630 mm³. With thegolf ball 2 in which the total volume TV is not less than 520 mm³,rising of the golf ball 2 during flight is suppressed. From thisviewpoint, the total volume TV is more preferably not less than 530 mm³and particularly preferably not less than 535 mm³. With the golf ball 2in which the total volume TV is not greater than 630 mm³, dropping ofthe golf ball 2 during flight is suppressed. From this viewpoint, thetotal volume TV is more preferably not greater than 620 mm³ andparticularly preferably not greater than 615 mm³.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight of ahigh-cis polybutadiene (trade name “BR-730”, manufactured by JSRCorporation), 22.5 parts by weight of zinc diacrylate, 5 parts by weightof zinc oxide, 5 parts by weight of barium sulfate, 0.5 parts by weightof diphenyl disulfide, and 0.6 parts by weight of dicumyl peroxide. Thisrubber composition was placed into a mold including upper and lower moldhalves each having a hemispherical cavity, and heated at 170° C. for 18minutes to obtain a core with a diameter of 38.5 mm.

A resin composition was obtained by kneading 50 parts by weight of anionomer resin (trade name “Himilan 1605”, manufactured by Du Pont-MITSUIPOLYCHEMICALS Co., Ltd.), 50 parts by weight of another ionomer resin(trade name “Himilan AM7329”, manufactured by Du Pont-MITSUIPOLYCHEMICALS Co., Ltd.), and 4 parts by weight of titanium dioxide witha twin-screw kneading extruder. The core was covered with this resincomposition by injection molding to form a mid layer with a thickness of1.6 mm.

A paint composition (trade name “POLIN 750LE”, manufactured by SHINTOPAINT CO., LTD.) including a two-component curing type epoxy resin as abase polymer was prepared. The base material liquid of this paintcomposition includes 30 parts by weight of a bisphenol A type solidepoxy resin and 70 parts by weight of a solvent. The curing agent liquidof this paint composition includes 40 parts by weight of a modifiedpolyamide amine, 55 parts by weight of a solvent, and 5 parts by weightof titanium dioxide. The weight ratio of the base material liquid to thecuring agent liquid is 1:1. This paint composition was applied to thesurface of the mid layer with a spray gun, and kept at 23° C. for 6hours to obtain a reinforcing layer with a thickness of 10 μm.

A resin composition was obtained by kneading 100 parts by weight of athermoplastic polyurethane elastomer (trade name “Elastollan XNY85A”,manufactured by BASF Japan Ltd.) and 4 parts by weight of titaniumdioxide with a twin-screw kneading extruder. Half shells were obtainedfrom this resin composition by compression molding. The sphereconsisting of the core, the mid layer, and the reinforcing layer wascovered with two of these half shells. These half shells and the spherewere placed into a final mold that includes upper and lower mold halveseach having a hemispherical cavity and having a large number of pimpleson its cavity face, and a cover was obtained by compression molding. Thethickness of the cover was 0.5 mm. Dimples having a shape that is theinverted shape of the pimples were formed on the cover. A clear paintincluding a two-component curing type polyurethane as a base materialwas applied to this cover to obtain a golf ball of Example 1 with adiameter of about 42.7 mm and a weight of about 45.6 g. The dimplepattern of the golf ball is shown in FIG. 2. The specifications of thedimples of the golf ball are shown in Table 1 below. Each dimple has acenter portion and an edge portion. The curvature radius CR of the edgeportion is 2.0 mm. In the dimple pattern, the ratio P1 is 100%, and theland ratio PL is 7.7%. The total volume TV of the dimples is 569 mm³.

Examples 2 to 13 and Comparative Example 2

Golf balls of Examples 2 to 13 and Comparative Example 2 were obtainedin the same manner as Example 1, except the final mold was changed andthe specifications of the dimples were set as shown in Tables 1 to 4below.

Comparative Example 1

A golf ball of Comparative Example 1 was obtained in the same manner asExample 1, except the final mold was changed and the specifications ofthe dimples were set as shown in Table 4 below. Each dimple of the golfball has an almost bowl shape.

[Flight Test]

A driver with a head made of a titanium alloy (trade name “XXIO-10”,manufactured by Sumitomo Rubber Industries, Ltd., shaft hardness: R,loft angle: 10.5°) was attached to a swing machine manufactured by GolfLaboratories, Inc. A golf ball was hit under the conditions of a headspeed of 40 m/sec, a launch angle of about 110, and a backspin rate ofabout 2200 rpm, and the distance from the launch point to the stop pointwas measured. During the test, the weather was almost windless. Theaverage value of data obtained by 20 measurements is shown in Tables 1and 4 below.

TABLE 1 Results of Evaluation Example 2 Example 1 Example 3 Front viewFIG. 6 FIG. 2 FIG. 7 P1 (%) 100 100 100 CR (mm) 2.5 2.0 1.5 PL (%) 5.67.7 12.1 TV (mm³) 589 569 549 Flight 197.8 197.2 196.2 distance (m)

TABLE 2 Results of Evaluation Example 4 Example 5 Example 6 Example 7Front view FIG. 8 FIG. 9 FIG. 10 FIG. 11 P1 (%) 100 100 100 100 CR (mm)2.5 2.0 1.5 1.0 PL (%) 3.8 5.8 9.6 12.5 TV (mm³) 602 592 577 562 Flight198.2 197.9 196.6 195.7 distance (m)

TABLE 3 Results of Evaluation Example 8 Example 9 Example 10 Example 11Front view FIG. 12 FIG. 13 FIG. 14 FIG. 15 P1 (%) 100 100 100 100 CR(mm) 2.5 2.0 1.5 1.0 PL (%) 1.9 3.9 7.0 9.0 TV (mm³) 631 620 609 598Flight 197.4 198.8 196.9 196.3 distance (m)

TABLE 4 Results of Evaluation Comp. Comp. Example 1 Example 2 Example 12Example 13 Front view FIG. 16 FIG. 17 FIG. 18 FIG. 19 P1 (%) 0 47 57 82CR (mm) — 1.5 1.5 1.5 PL (%) 18 16.2 15.2 12.6 TV (mm³) 567 519 527 552Flight 195.0 194.5 195.7 196.0 distance (m)

As shown in Tables 1 to 4, the golf ball of each Example has excellentflight performance. From the evaluation results, advantages of thepresent invention are clear.

The aforementioned dimple pattern is applicable to golf balls havingvarious structures such as a one-piece golf ball, a two-piece golf ball,a four-piece golf ball, a five-piece golf ball, a six-piece golf ball, athread-wound golf ball, and the like in addition to a three-piece golfball. The above descriptions are merely illustrative examples, andvarious modifications can be made without departing from the principlesof the present invention.

What is claimed is:
 1. A golf ball having a plurality of dimples on asurface thereof, wherein each dimple has a center portion having a bowlshape, and an edge portion smoothly continuous with the center portionand having a ring shape, a cross-sectional shape of the edge portion isa curved line that is convex outward in a radial direction of the golfball, and a ratio P1 of a number N1 of the dimples each having an edgeportion that is adjacent to another edge portion such that the edgeportion shares a boundary line with the other edge portion, to a totalnumber N of the dimples, is not less than 50%.
 2. The golf ballaccording to claim 1, wherein a land ratio PL of the surface of the golfball is not greater than 12%.
 3. The golf ball according to claim 1,wherein a cross-sectional shape of the edge portion is a circular archaving a curvature radius CR of not less than 1.25 mm.
 4. The golf ballaccording to claim 1, wherein a total volume of the dimples is not lessthan 520 mm³ and not greater than 630 mm³.